1. Field of the Invention
This invention relates to a catalyst for the aromatization of alkanes to aromatics, specifically a zeolite, preferably a MFI-type structure, most preferably a ZSM-5 MFI zeolite, catalyst for the aromatization of alkanes having two to six carbon atoms per molecule to aromatics, such as benzene, toluene and xylene.
2. Description of the Prior Art
Zeolite is a crystalline hydrated aluminosilicate that may also contain other metals, such as sodium, calcium, barium, and potassium, and that has ion exchange properties (Encarta® World English Dictionary [North American Edition] 201 & (P) 2001 Microsoft Corporation). A method for preparing a zeolite comprises (a) preparing an aqueous mixture of silicon oxide and sources of oxides of aluminum; and (b) maintaining said aqueous mixture under crystallization conditions until crystals of said zeolite form. Much zeolite research has focused on the synthesis of zeolite frameworks containing elements other than silicon and aluminum.
U.S. Pat. No. 6,160,191 discloses that the term “zeolite” includes not only aluminosilicates but substances in which the aluminum is replaced by gallium, titanium, iron or boron and substances in which silicon is replaced by germanium, tin and phosphorous. U.S. Pat. Nos. 3,329,480 and 3,329,481, both issued to D. A. Young, report the existence of crystalline zirconosilicate and titanosilicate zeolites. A zeolite having chromium in the tetrahedral positions has been described by Yermolenko et al at the Second Oil Union Conference on Zeolites, Leningrad, 1964, pages 171-8 (published 1965). However, D. W. Breck, in Zeolite Molecular Sieves, p. 322, John Wiley & Sons (1974) suggests that the chromium present was not present in a zeolite A structure and furthermore was present as an impurity in insoluble form. The impurity was said to be in the form of a chromium silicate as confirmed by the nature of the water vapor adsorption isotherm.
The zeolite ZSM-5 has been synthesized with many elements other than Al in the framework, including iron. Synthesis of an iron-containing zeolitic structure was reported in Japanese Kokai 59,121,115, published Jul. 13, 1984, which disclosed an aluminosilicate having a faujasite structure and containing coordinated iron. The chemical composition is said to be of the formula aM2/nO:bFe2O3:Al2O3:cSiO2 where M can be H, alkali metal or alkaline earth metal; the symbol n is the valence of M; a=1+/−0.3; c is between 4.6 and 100; and a is less than b and both are less than 7. The crystal lattice parameter ao is between 24.3 and 24.7 angstroms. Similarly, U.S. Pat. No. 4,208,305 discloses crystalline silicates which are structurally a three-dimensional network of SiO42−, FeO42− and, optionally, AlO4−, GaO431  and GeO4 − tetrahedrons, which are interlinked by common oxygen atoms and are of the overall composition in dehydrated form:(1.0+/−0.3)(R)2/nO[aFe2O3bAl2O3cGa2O3]y(d SiO2eGeO2),where R is a cation; a≧0.1; b≧0; c≧0; a+b+c=1; y≧10; d≧0.1; e≧0; d+e=1; and n is the valence of R. Silicates not containing gallium, germanium and aluminum are preferred. Silicates of a particular X-ray powder diffraction pattern are also preferred.
U.S. Pat. No. 4,713,227 discloses crystalline metalloaluminophosphates having microposity, catalytic activity and ion-exchange properties which contain metals such as arsenic, bismuth, cobalt, iron, germanium, manganese, vanadium and antimony within the framework.
U.S. Pat. No. 5,179,054 states that although matrices may be formed with the germanium analog of silicon, these are expensive and generally no better than the metallosilicates, i.e., aluminosilicate, gallosilicate, ferrosilicate and borosilicate, for the molecular sieve component of a catalyst for catalytic cracking of heavy hydrocarbon oils to produce hydrocarbons boiling in the gasoline and distillate range.
In the publication “Aromatization of butane on Pt—Ge intermetallic compounds supported on HZSM-5” by T. Komatsu et al in Applied Catalysis A: General, vol. 194-195, p. 333-339 (2000) platinum-germanium intermetallic species, platinum species and germanium species were deposited on HZSM-5 for catalysts to convert butane to aromatics. The Pt—Ge intermetallic catalyst was reported to have a more stable time on stream for conversion and selectivity than that of the Pt catalyst. Catalysts with large amount of germanium relative to platinum were reported to have lower conversion of butane and lower selectivity to aromatics.
U.S. Pat. No. 4,704,494 discloses a process for conversion of low molecular paraffin hydrocarbons to aromatic hydrocarbons using a platinum- or gallium-modified metallosilicate (Si/Me) catalyst where Me is aluminum, gallium, titanium, zirconium, germanium, lanthanum, manganese, chromium, scandium, vanadium, iron, tungsten, molybdenum, nickel or a mixture thereof. The working examples were prepared with Me being aluminum or gallium. No example with germanium was prepared. There was no suggestion to select germanium over any of the other elements or mixtures of elements.
U.S. Pat. No. 5,456,822 discloses a process for aromatization of hydrocarbons containing two to nine carbon atoms per molecule with a catalyst containing an MFI zeolite having silicon, aluminum and/or gallium in the framework, a matrix, and gallium, a noble metal of the platinum family and a metal selected from tin, germanium, indium, copper, iron, molybdenum, gallium, thallium, gold, silver, ruthenium, chromium, tungsten and lead deposited on the zeolite. No example with germanium was prepared. There was no suggestion to select germanium over any of the other elements or mixtures of elements.
U.S. Pat. Nos. 4,910,357 and 5,124,497 disclose a process for producing monosubstituted monoalkylaromatics from C8+ paraffins using a nonacid platinum-containing catalyst in which the crystalline material contains tin, indium, thallium or lead. These catalysts provided much higher selectivity to aromatics than other platinum catalysts incorporating other elements in the crystalline material, including germanium.
U.S. Pat. No. 6,315,892 discloses a process for making a catalyst of a carrier, platinum and germanium wherein the germanium is introduced as an organic compound which is contacted with the precatalyst in the reaction zone.
U.S. Pat. No. 5,227,557 discloses a process for the aromatization of hydrocarbons containing 2 to 4 carbon atoms per molecule using an MFI zeolite catalyst containing platinum and one metal chosen from tin, germanium, lead and indium. Both platinum and the additional metal can be introduced on the MFI zeolite by impregnation, exchange or other known methods. The working examples used impregnation for both platinum and the additional metal. Example E contained 0.3% platinum and 0.2% germanium.
U.S. Pat. No. 4,036,741 discloses a process for converting hydrocarbons (including dehydrocyclization of paraffins to aromatics) with an acidic catalyst of a porous carrier material containing halogen along with platinum, cobalt and germanium uniformly dispersed throughout the carrier material which may be a crystalline zeolitic aluminosilicate. The catalyst was tested as a reforming catalyst for a relatively low-octane gasoline fraction under substantially sulfur-free conditions against a catalyst containing platinum and germanium but without cobalt. The results indicated that cobalt was essential for better activity and activity stability.
Some zeolite catalysts containing a Group VIII metal are susceptible to sulfur poisoning. For some platinum catalysts, despite some sensitivity to sulfur, modest amounts of sulfur, such as 10 to 100 ppm, are acceptable and sometimes preferred. A standard sulfurization method that is well known in the art consists in heating in the presence of hydrogen sulfide or a mixture of hydrogen sulfide and hydrogen or nitrogen to a temperature of between 150 and 800° C., preferably between 250 and 600° C.
U.S. Pat. No. 4,836,336 discloses a process for converting C6-C12 paraffinic hydrocarbon feed to aromatics using a noble metal low acidity medium pore size zeolite catalyst. The noble metal is preferably a platinum group metal (platinum, palladium, iridium, osmium, rhodium or ruthenium) and the zeolite is preferably ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-38, ZSM-48 or Zeolite Beta. The noble metal is modified to its sulfide form by presulfiding the catalyst with H2S, SO2 or an organic sulfur compound to increase aromatic selectivity of the catalyst. Germanium is not disclosed as a component of the catalyst.
It would be advantageous to have a zeolite-type catalyst which maintained relatively constant selectivity for conversion of lower alkanes to aromatics, such as benzene, toluene and xylene, over a period of time on stream.